Methods and apparatus for tuning an LC filter

ABSTRACT

A receiver, such as a television receiver, tunes an inductive (“L”) and capacitive (“C”) filter based on a desired frequency. The LC filter includes a plurality of inductors, configured in at least one inductive (“L”) bank, and a plurality of capacitors configured in at least one capacitive (“C”) bank. The inductors are selectively enabled for the LC filter by an N code, and the capacitors are selectively enabled for the LC filter by an M code. The receiver uses a functional comparator to compare offset, capacitive, inductive and frequency values to tune the LC filter. Techniques for tuning the LC filter for operation in the VHF and UHF bands are disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/386,644, filed Jun. 5, 2002, entitled “FrequencyDiscrete L-C Tank for TV Tuner”, U.S. Provisional Patent Application No.60/386,471, filed Jun. 5, 2002, entitled “Functional Comparator forBinary L-C bank Addressing,” and U.S. Provisional Patent Application No.60/384,290, filed May 29, 2002, entitled “Fast Tuning Algorithm for TVReception.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed toward the field of discrete passivefilters, and more particularly toward a tunable LC filter bank.

2. Art Background

Typically, receivers employ filters to condition both input signals andinternally generated reference signals. For example, bandpass, notch,and low pass are types of filters employed in receivers. The frequencyresponse of a filter refers to the characteristics of the filter thatcondition the signal input to the filter. For example, a bandpass filtermay attenuate an input signal across a pre-determined band offrequencies above and below a center frequency of the filter. Filtersare designed to exhibit frequency responses based on one or more circuitparameters.

Some receivers are designed to process input signals with a range ofinput carrier frequencies (e.g., wide band receivers). For example,television receivers must be capable of processing input televisionsignals with carrier frequencies ranging from 55 MHz to 880 MHz. Onecircuit parameter used to define the frequency response of a filter isthe carrier frequency of an input signal. Thus, such wide band receiversrequire filters to generate multiple frequency responses to accommodatemultiple input carrier frequencies. To accomplish this, some receiversemploy tunable filters to process a wide band of input frequencies.

One type of tunable filter is a varactor type tuner. A popularapplication for the varactor is in electronic tuning circuits, such astelevision tuners. A direct current (“DC”) control voltage varies thecapacitance of the varactor, re-tuning a resonant circuit (i.e.,filter). Specifically, a varactor diode uses a pn junction in reversebias such that the capacitance of the diode varies with the reversevoltage. However, the relationship between the control voltage and thecapacitance in a varactor tuner is not linear. Thus, the capacitancevalue is based on the signal level. This non-linearity producesdistortion in the output of the filter (e.g., a third order product).

Other receivers, such as television receivers, may employ activefilters. The use of a continuous or active filter requires a powersupply voltage (e.g., V_(cc)). The power supply voltage exhibits aripple due to noise on the voltage supply line. This ripple voltage, inturn, causes unacceptable frequency response characteristics on theoutput of the continuous amplifier. Accordingly, it is desirable to usediscrete or passive filters in the receiver to isolate the signal fromripple voltage, thereby improving signal quality.

SUMMARY OF THE INVENTION

A receiver, such as a television receiver, tunes an inductive (“L”) andcapacitive (“C”) filter based on a desired frequency. The LC filterincludes a plurality of inductors, configured in at least one inductive(“L”) bank, and a plurality of capacitors configured in at least onecapacitive (“C”) bank. The inductors are selectively enabled for the LCfilter by an N code, and the capacitors are selectively enabled for theLC filter by an M code.

In order to tune the receiver for a channel frequency in the VHFspectrum, the receiver determines a first offset between an initialresonant frequency of the LC filter and the channel frequency (i.e., thedesired frequency for which the LC filter is tuned). For course tuning,the N code is determined based on an initial value for the M code, thedesired frequency, and the first offset. At least one inductor in the Lbank is selected based on the N code. Then, a second offset between aresonant frequency of the LC filter and the desired frequency isdetermined. For fine-tuning, the M code is determined based on the valuefor the N code, the desired frequency, and the second offset. At leastone capacitor in the C bank is selected using the M code.

In order to tune the receiver for a channel frequency in the UHFspectrum, the receiver also determines a first offset between an initialresonant frequency of the LC filter and the desired frequency. Forcourse tuning, the M code is determined based on an initial value forthe N code, the desired frequency, and the first offset. At least onecapacitor in the C bank is selected from the M code. A second offsetbetween a resonant frequency of the LC filter and a desired frequency isdetermined. To fine-tune the LC filter, the N code is determined basedon the value for the M code, the desired frequency, and the secondoffset. At least one inductor in the L bank is selected from the N code.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment for a televisiontuner that utilizes LC bank filters.

FIG. 2 is a block diagram illustrating one embodiment for implementingthe LC bank filters in a television tuner.

FIG. 3a illustrates one embodiment for an inductive (L) bank for use inthe LC filter bank.

FIG. 3b illustrates one embodiment for a capacitive bank for use in theLC filter bank of the present invention.

FIGS. 4a and 4 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the VHF spectrum.

FIG. 5 shows one embodiment for selecting inductors in an inductor bankfrom the N code.

FIG. 6 illustrates various resistances for selected inductances of the Lbank.

FIG. 7 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total capacitance as a functionof the M code.

FIG. 8 depicts relationships between the selected M code and centerfrequency for various combinations of the N code.

FIG. 9 shows the information for capacitance and M code for selectingcapacitors in a C Bank during VHF tuning.

FIG. 10 shows various resistances for selected capacitances of the Cbank.

FIGS. 11a and 11 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the UHF spectrum.

FIG. 12 shows one embodiment for selecting capacitors in a capacitorbank for UHF tuning.

FIG. 13 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total inductance as a function ofthe N−1 code.

FIG. 14 depicts relationships between the selected N−1 code and centerfrequency for various combinations of the M code.

FIGS. 15a and b show the information for selecting inductors in an LBank during UHF tuning.

FIG. 16 illustrates various resistances for selected capacitances of theC bank.

FIG. 17 is a timing diagram that shows timing for tuning the LC filterbank in accordance with one embodiment.

FIG. 18 illustrates one embodiment for a functional comparator circuitused in one embodiment for tuning the LC filter bank.

FIG. 19 illustrates one embodiment for a calculator used in thefunctional comparator circuit of FIG. 18.

FIG. 20 illustrates a plurality of frequency responses for oneembodiment of the LC filter bank.

DETAILED DESCRIPTION

The disclosures of U.S. Provisional Patent Application No. 60/386,644,filed Jun. 5, 2002, entitled “Frequency Discrete L-C Tank for TV Tuner”,and U.S. Provisional Patent Application No. 60/386,471, filed Jun. 5,2002, entitled “Functional Comparator for Binary L-C bank Addressing”are hereby expressly incorporated herein by reference.

The present invention includes one or more inductive (“L”) andcapacitive (“C”) filter banks applied to realize a non-varactor typetelevision tuner. In one embodiment, the television tuner is integratedinto a single integrated circuit chip. The LC banks are used toimplement passive filters. The television tuner optimally selectscombinations of inductors and capacitors in the LC bank to tune thetelevision receiver.

FIG. 1 is a block diagram illustrating one embodiment for a televisiontuner that utilizes LC bank filters. The television tuner 100 receives aradio frequency (“RF”) television signal, and generates demodulatedbaseband television signals (i.e., picture and sound signals). For thisembodiment, television tuner 100 includes inductive banks 110 and 124,as well as capacitive banks 115 and 126. Inductive bank 110 andcapacitive bank 115 comprise LC filter bank 112. Similarly, inductivebank 124 and capacitive bank 126 constitute LC filter bank 125. Asdescribed fully below, LC filter banks 110 and 115 provide a band passfilter function for television receiver 2100.

The television circuit 100 also includes inductors 102 and 104 to filterthe input RF signal. For this embodiment, the inductors 102 and 104values are set to 21.8 nano henries (“nH”) and 91.2 nH, respectively. Anautomatic gain control circuit 120 amplifies the signal, output from LCfilter bank 112, for input to the second LC filter bank 125. Inductor122, with a value of 91.2 nH, adds a parallel inductance to LC filterbank 125. As described more fully below, LC filter banks 112 and 125generate a band pass frequency response for conditioning of the inputsignal.

The television tuner 100 contains one or more down conversion stages.For this embodiment, television tuner 100 includes two quadraticdownconverters. A first quadratic downconverter is implemented usingmixers 130 and 132, local oscillator 140, and notch filter 150. Thefirst quadratic downconverter converts the frequency of the filtered RFtelevision signal to a first immediate frequency (e.g., 45.75 mega hertz(“MHz”)). In general, a quadratic demodulator splits the input signal,and mixes the input signal with an in-phase (“I”) local oscillatorsignal and a quadrature phase (“Q”) local oscillator signal. The Q localoscillator signal is phase shifted 90 degrees from the I localoscillator signal.

A second quadratic downconverter circuit, which receives the outputsignal from the first quadratic downconverter circuit, includes mixers160 and 165, local oscillator 170, notch filter 180, and band passfilter 190. The second quadratic downconverter converts the frequency ofthe first intermediate television signal to a second immediate frequency(e.g., 10.5 mega hertz (“MHz”)).

The television receiver also includes IF processing 195. The IFprocessing module 195 generates baseband picture and sound carriercomponents. One embodiment for IF processing that utilizes a quadraticdemodulator to process a television signal is described in U.S. patentapplication Ser. No. [EXPRESS MAIL NO.: EL 909174649], entitledQuadratic Nyquist Slope Filter, filed Sep. 30, 2002, which is expresslyincorporated herein by reference.

FIG. 2 is a block diagram illustrating one embodiment for implementingthe LC bank filters in a television tuner. A television receiver 200includes inductive “L” banks A and B. For this embodiment, the L banksare implemented external to an integrated circuit 200. The L bank “A”consists of five inductors (202, 204, 206, 208, and 209). Similarly, Lbank “B” contains five inductors (212, 214, 216, 218, and 219). Eachinductor of inductive bank A is electrically coupled to integratedcircuit 200 through an input/output (“I/O”) pad (i.e., pad A1 couplesconductive 202, pad A2 couples inductors 204, pad A3 couples inductive206, pad A4 couples inductor 208, and pad A5 couples inductor 209).Similarly, an I/O pad is provided for each inductor in L bank B (i.e.,pad B1 couples inductor 212, pad B2 couples inductor 214, pad B3 couplesinductor 216, pad B4 couples inductor 218, and pad B5 couples inductor219). A switch is provided for each inductor in both L banks A and B(switches 203, 205, 207, 208 and 211 for L bank A, and switches 213,215, 217, 221 and 223 for L bank B). A total inductance is generated foreach L bank by selectively coupling the external inductors to thetelevision receiver circuit through the switches. Specifically, adigital code (hereafter referred to as the N code) is applied to theswitches to select inductors in the L bank. In one embodiment, theswitches are implemented using metal oxide semiconductor (“MOS”)transistors.

L bank A has a corresponding capacitive (“C”) bank A, labeled 220 inFIG. 2. Similarly, L bank B has a corresponding capacitive bank B,labeled 210 in FIG. 2. In one embodiment, the C bank contains fourcapacitors per bank. As shown in FIG. 2, C bank 220 includes a pluralityof capacitors (225, 222, 224, and 227) coupled to a plurality ofswitches (232, 231, 229 and 228). The capacitors are selected using acode (hereafter referred to as an M code), to control the switches thatcouple the capacitors to the television receiver circuit.

The television receiver circuit 200 includes circuitry to select orprogram the LC filter banks. In general, television receiver 200generates the M code and N code to selectively program the LC filterbanks. By selecting different combinations of inductors (L bank) andcapacitors (C bank), different filter characteristics (i.e., frequencyresponses) are realized. For the embodiment shown in FIG. 2, televisionreceiver 200 receives information, referred to as a channel code, thatspecifies the desired television channel. In one embodiment, the channelcode is received by bus transceiver 270, using its external pins (ENB,SCL, and SDA). A programmable divider 240 is used to digitize at least aportion of the channel code (e.g., 10 bits). The programmable divider240 is also used to digitize an LC oscillation frequency. The LCoscillation frequency is the center frequency for a tuned cavitygenerated by the combination of selected inductors and capacitors fromthe LC banks. The LC oscillation frequency is generated using amplifier(AGC) 230. The tuning circuit for television receiver 200 includes atiming reference, and associated circuitry to generate various timingsignals. An oscillator circuit, which uses a 16 MHz crystal (255), iscoupled to a divider 250 to generate timing signals.

Television receiver 200 also includes a plurality of digital to analog(D/A) circuits 262 to convert digital values to analog currents. In oneembodiment, the analog currents are used in a calculator 264 and acomparator circuit 268 for tuning of the LC filter banks. Register 272stores a digital value, A. The digital values for M code and N code arestored in registers 274 and 276, respectively. As shown in FIG. 2,digital values (A, M, N and F_(LC), and frequency of the channel code“F_(ch)”) are input to D/A circuits 262. As shown in FIG. 2, thecalculator 264 and comparator circuit 268 generates a value for A, the Mcode and the N code using the timing from timing circuit 260.

FIG. 3a illustrates one embodiment for an inductive (L) bank for use inthe LC filter bank. For this embodiment, the inductive bank includesfive inductors (315, 320, 325, 330 and 340). Although the inductive bank300 includes five inductors, any number of inductors may be used withoutdeviating from the spirit or scope of the invention. In one embodiment,the number and values for the inductors is a function of the desiredfrequency response characteristics of the LC filter bank. The inductors,which form inductive bank 300, are configured in parallel. For theembodiment of FIG. 3a, the inductor values are 5.7, 11.4, 22.8, 45.6,and 91.2 nH. Each inductor is added to the L bank through acorresponding switch (switches 310, 308, 306, 304 and 302). In oneembodiment, the switches are implemented using metal oxide semiconductor(“MOS”) transistors.

FIG. 3b illustrates one embodiment for a capacitive bank for use in theLC filter bank of the present invention. For this embodiment, capacitivebank 350 contains five capacitors (360, 362, 364, 366 and 368). For thisembodiment, the capacitor values are 3.7, 9.4, 17, 32.8 and 64.6 pF. Adifferent number of capacitors and different capacitive values may beselected to implement filters for the LC filter bank with differentfrequency responses. Also, as shown in FIG. 3b, capacitors 360, 362, 364and 366 are selected for the C bank through switches 358, 356, 354 and352, respectively. In one embodiment, the switches are implemented withMOS transistors.

FIGS. 4a and 4 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the VHF spectrum. The processis initiated by selecting an initial value for the inductance, L, (i.e.,N code) and capacitance, C (i.e., M code) (block 400, FIG. 4a). In oneembodiment, M is set to a value of binary 0100, and N is set to a valueof binary 0001. A variable, A, is used to determine an offset betweenthe currently tuned LC oscillator frequency, F_(LC), and the desiredtuned channel frequency, F_(ch). In one embodiment, A comprises a fivebit digital value. The variable, A, is set to an initial condition(block 410, FIG. 4). In one embodiment, the initial condition is equalto “10000.”

The circuit obtains a sample value for the LC oscillator frequency(“F_(LC)”) (block 420, FIG. 4a). The LC oscillator frequency isconverted from an analog signal to a digital value using a high-speeddivider. The comparator circuit (285, FIG. 2) is used to evaluate theexpression:

CF _(LC) ² =A ²(1/L)

In one embodiment, digital values for C (i.e., M code), F_(LC), A, and L(i.e., N code) are converted to analog currents using a digital toanalog converter, and the analog currents are input to a calculator, togenerate both numeric sides of the expression, and a comparator toevaluate the expression (See FIGS. 11 & 12). If the expression is nottrue (i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is greaterthan A²(1/L), then the digital value for A is incremented (blocks 425,427 and 430, FIG. 4a). For example, after the first evaluation, ifCF_(LC) ² is greater than A²(1/L), then the initial value of A, 10000,is incremented to the value of 10001. If the expression is not true(i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is less thanA²(1/L), then the digital value for A is decremented (blocks 425, 427and 428, FIG. 4a). For example, after the first evaluation, if CF_(LC) ²is less than A²(1/L), then the initial value of A, 10000, is decrementedto the value of 01111. This process repeats until the expressionevaluates to true. When the expression evaluates to true, the offset, A,has been calculated. The offset is stored in a register for subsequentuse (block 440, FIG. 4a).

To tune the input VHF channel, a value for the N code is firstdetermined. The selection of N (inductive selection) results in coarsetuning a channel in the VHF spectrum. The circuit sets an initial valuefor the N code (block 450, FIG. 4a). In one embodiment, N is set to avalue of 00001. Also, values are selected for M and F_(ch) (block 455,FIG. 4a). In one embodiment, the M code is set to a value of 0100, andF_(ch) is set to the value of the channel code (i.e., the desired tunedfrequency).

A circuit evaluates the expression:

CF _(ch) ² =A ²(1/L).

In one embodiment, digital values for C (i.e., M code), F_(ch), A, and L(i.e., N code) are converted for evaluation of the expression. If theexpression is not true (i.e., CF_(ch) ² does not equal A²(1/L)), thenthe digital value for N is incremented (blocks 465 and 470, FIG. 4a).For example, after the first evaluation, if CF_(LC) ² is not equal toA²(1/L), then the initial value of N, 00001, is incremented to the valueof 00010. This process repeats until the expression evaluates to true.When the expression evaluates to true, the N code has been determined,and the L bank is set based on the N code (block 455, FIG. 4a).

The process re-calculates the offset, A, after the course tuningprocedure. The variable, A, is set to an initial condition (e.g., 10000)(block 462, FIG. 4b). The circuit obtains a new sample value for the LCoscillator frequency, F_(LC), after the course tuning (block 464, FIG.4b). The frequency, F_(LC), is converted from an analog signal to adigital value using a high-speed divider. The following expression isevaluated:

CF _(LC) ² =A ²(1/L)

If the expression is not true (i.e., CF_(LC) ² does not equal A²(1/L)),and CF_(LC) ² is greater than A²(1/L), then the digital value for A isincremented (blocks 466, 468 and 470, FIG. 4b). If the expression is nottrue (i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is lessthan A²(1/L), then the digital value for A is decremented (blocks 466,468 and 469, FIG. 4b). This process repeats until the expressionevaluates to true. When the expression evaluates to true, the newoffset, A, has been calculated. The offset is stored in a register forsubsequent use (block 472, FIG. 4b).

Next, a value for the M code, and consequently C, is determined. Theselection of M results in fine tuning a channel in the VHF spectrum.First, an initial value is set for M (block 474, FIG. 4b). In oneembodiment, M is initially set to a value of 0000. Also, the values forF_(ch) and N code are selected. F_(ch) represents the frequency of thechannel code. The N code was set from the coarse tuning stage. Theexpression, F_(ch) ^(1.5)C=A^(1.5)(1/L), is evaluated to determine if itis true (block 478, FIG. 4b). In one embodiment, these digital valuesare converted to analog current values using digital to analogconverters. The analog current values are then weighted in accordancewith the expression (e.g., A^(1.5), F_(ch) ^(1.5), etc.) using acalculator, and then compared using a comparator (See FIGS. 11 and 12).If the expression does not evaluate to true, the M code is incremented(block 480, FIG. 4b), and the expression is again evaluated with the newM code. This process is repeated until the expression evaluates to true.With the expression evaluates to true, the capacitive bank is set basedon the M code (block 490, FIG. 4b).

FIG. 5 shows one embodiment for selecting inductors in an inductor bankfrom the N code. The first column of FIG. 5 shows inductor values forthe corresponding inductors in the inductor bank shown in column 2. Forexample, inductor L5 has a value of 5.7 nH, while inductor L1 has avalue of 91.2 nH. Note that inductor L0 is always selected. A binary Ncode for selecting inductors in the inductor bank is shown. For example,column 3 shows selection of inductor L0 for the N code of 00000. Thelast column of FIG. 5 shows selection of inductors from the inductorbank for the N code of 00111. Specifically, the “00111” N code specifiesselection of inductors L0, L1, L2 and L3 from the inductor bank. Thelast row of FIG. 5 shows the total inductance for the corresponding Ncode. For example, for the “00000” N code, the total inductance of theinductance bank is equal to 91.2 nH. The total inductance for the“00111” N code is equal to 11.4 nH.

The inductors and capacitors are selected from the C and L banks,respectively, the use of switches (e.g., MOS transistors). A resistanceis introduced into the LC bank filter response by each transistor. Thus,each capacitor selected in the C bank increases the series resistance.The increase in series resistance, or decrease in parallel resistance,decreases the Q factor, which, in turn, degrades performance of the LCbank filter.

In general, a Q factor is measured based on the expression:

Q=2¶fRC

In one embodiment, the receiver selects a combination of inductors andcapacitors to configure an LC filter bank so as to maximize the Qfactor. As shown by the above expression, the larger the parallelresistance and capacitance, the greater the Q factor. It is an objectiveto maximize the Q factor through proper selection of inductance andcapacitance combinations from the LC banks.

FIG. 6 illustrates various resistances for selected inductances of the Lbank. Specifically, column two lists a resistance, r_(sl), from thecorresponding inductor. For example, L5 has a resistance of 0.7 ohms.The third column lists the resistance, R_(MOS), from the MOS transistorfor the corresponding inductor. For example, L4 has a R_(MOS) resistanceof 1.6 ohms. The fourth column lists the total series resistance, r_(s),from selecting the corresponding inductor. For example, L3 has a totalseries resistance, r_(s), of 2.8 ohms. The last row of FIG. 6 lists thetotal series resistance for the corresponding N code. For example, an Ncode of 00010 has a total series resistance of 2.4 ohms. The circuitselects combinations of the inductors from an L bank to reduce the totalseries resistance and thus maximize the Q factor.

FIG. 7 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total capacitance as a functionof the M code. As shown in FIG. 7, for center frequencies in the VHFspectrum, a higher M code (i.e., overall capacitance) yields a higherfrequency. FIG. 7 also depicts the relationship between the M code andcenter frequency of the LC bank filter (and the desired channel) foreach value of the N code.

In order to maximize the Q factor, only certain combinations of the Ncode and M code are used. FIG. 8 depicts relationships between theselected M code and center frequency for various combinations of the Ncode. Note that the relationship between the center LC bank filterfrequency and the M code is now confined to the center of a graph. Thisrelationship minimizes the Q factor, and optimizes the response of theLC filter bank.

As discussed above, the tuning circuit of the present invention onlyselects certain combinations of N and M codes to configure the LC bankfilter. FIG. 9 shows the information for capacitance and M code forselecting capacitors in a C Bank during VHF tuning. The first column ofFIG. 9 lists the total capacitance, which ranges from 3.7 pF to 127.7pF, for the C bank. The second column identifies an M code to obtain thecorresponding capacitance value in column one. For example, a decimal Mcode of 12 yields a total capacitance of 101.3 pF. Column 3 of FIG. 9shows valid selections for the M code based on the frequency and thevalue of the N code. For example, in column 3, a list of frequencies isshown for valid selections of the M code when the N code is equal to 1.Specifically, column 3 shows that for these frequencies and N code, theM code has a valid range between 8 and 16 (decimal).

FIG. 10 shows various resistances for selected capacitances of the Cbank. Column two lists a resistance, R_(MOS), for a correspondingcapacitor. For example, M code 10 (decimal) has a resistance of 1.8ohms. The circuit selects combinations of the capacitors from a C bankto reduce the total series resistance and thus maximize the Q factor.

FIGS. 11a and 11 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the UHF spectrum. The processis initiated by selecting an initial value for the inductance, L, (i.e.,N code) and capacitance, C (i.e., M code) (block 700, FIG. 11a). In oneembodiment, M is set to a value of binary 0001, and N is set to a valueof binary 10000. The variable, A, is set to an initial condition (block710, FIG. 11a). In one embodiment, the initial condition is equal to“10000.”

The circuit obtains a sample value for the LC oscillator frequency(“F_(LC)”) (block 720, FIG. 11a). The LC oscillator frequency isconverted from an analog signal to a digital value using a high-speeddivider. The following expression is evaluated:

CF _(LC) ² =A ²(1/L).

If the expression is not true (i.e., CF_(LC) ² does not equal A²(1/L)),and CF_(LC) ² is greater than A²(1/L), then the digital value for A isincremented (blocks 725, 727 and 730, FIG. 11a). For example, after thefirst evaluation, if CF_(LC) ² is greater than A²(1/L), then the initialvalue of A, 10000, is incremented to the value of 10001. If theexpression is not true (i.e., CF_(LC) ² does not equal A²(1/L)), andCF_(LC) ² is less than A²(1/L), then the digital value for A isdecremented (blocks 725, 727 and 728, FIG. 11a). For example, after thefirst evaluation, if CF_(LC) ² is less than A²(1/L), then the initialvalue of A, 10000, is decremented to the value of 01111. This processrepeats until the expression evaluates to true. When the expressionevaluates to true, the offset, A, has been calculated. The offset isstored in a register for subsequent use (block 740, FIG. 11a).

To tune the input UHF channel, a value for the M code is firstdetermined. The selection of M (capacitive selection) results in coarsetuning a channel in the UHF spectrum. The circuit sets an initial valuefor the M code (block 750, FIG. 11a). In one embodiment, M is set to avalue of 00001. Also, values are selected for N and F_(ch) (block 755,FIG. 11a). The N code is set to a value of 10000, and F_(ch) is set tothe value of the channel code (i.e., the desired tuned frequency).

A circuit evaluates the expression:

CF _(Ch) ^(1.5)=2(A ^(1.5)(1/L)).

If the expression is not true (i.e., CF_(ch) ^(1.5) does not equal2(A^(1.5)(1/L))), then the digital value for M is incremented (blocks765 and 770, FIG. 11a). For example, after the first evaluation, ifCF_(ch) ^(1.5) is not equal to 2(A^(1.5)(1/L)) then the initial value ofM, 00001, is incremented to the value of 00010. This process repeatsuntil the expression evaluates to true. When the expression evaluates totrue, the M code has been determined, and the C bank is set based on theM code (block 755, FIG. 11a).

The process re-calculates the offset, A, after the course tuningprocedure. The variable, A, is set to an initial condition (e.g., 10000)(block 762, FIG. 11b). The circuit obtains a new sample value for the LCoscillator frequency, F_(LC), after the course tuning (block 764, FIG.11b). The frequency, F_(LC), is converted from an analog signal to adigital value using a high-speed divider. The following expression isevaluated:

CF _(LC) ² =A ²(1/L)

If the expression is not true (i.e., CF_(LC) ² does not equal A²(1/L)),and CF_(LC) ² is greater than A²(1/L), then the digital value for A isincremented (blocks 766, 768 and 770, FIG. 11b). If the expression isnot true (i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is lessthan A²(1/L), then the digital value for A is decremented (blocks 766,768 and 769, FIG. 11b). This process repeats until the expressionevaluates to true. When the expression evaluates to true, the newoffset, A, has been calculated. The offset is stored in a register forsubsequent use (block 772, FIG. 11b).

Next, a value for the N code, and consequently L, is determined. Theselection of the N code results in fine tuning a channel in the UHFspectrum. First, an initial value is set for N (block 774, FIG. 11b). Inone embodiment, N is initially set to a value of 0000. Also, the valuesfor F_(ch) and M code are selected. F_(ch) represents the frequency ofthe channel code. The M code was set from the coarse tuning stage. Theexpression, F_(ch) ^(2.0)C=A^(2.0)(1/L), is evaluated to determine if itis true (block 778, FIG. 11b). In one embodiment, these digital valuesare converted to analog current values using digital to analogconverters. The analog current values are then weighted in accordancewith the expression (e.g., A^(2.0), F_(ch) ^(2.0), etc.) using acalculator, and then compared using a comparator (See FIGS. 11 and 12).If the expression does not evaluate to true, the N code is incremented(block 780, FIG. 11b), and the expression is again evaluated with thenew N code. This process is repeated until the expression evaluates totrue. With the expression evaluates to true, the inductive bank is setbased on the N code (block 790, FIG. 11b).

FIG. 12 shows one embodiment for selecting capacitors in a capacitorbank for UHF tuning. The first column of FIG. 12 shows capacitor valuesfor the corresponding capacitors listed in column 2. For example,capacitor C3 has a value of 32.8 pF, while capacitor C1 has a value of9.4 pF. Note that capacitor C0 is always selected. A binary M−1 code forselecting capacitors in a capacitor bank is shown. For example, column 3shows selection of capacitor C0 for the M−1 code of 00001. The lastcolumn of FIG. 12 shows selection of capacitors from the capacitor bankfor the M−1 code of 01111. Specifically, the “01111” M−1 code specifiesselection of capacitors C0, C1, C2 and C3 from the capacitor bank. Thelast row of FIG. 12 shows the total capacitance for the various M−1codes. For example, for the “00001” M−1 code, the total capacitance ofthe capacitor bank is equal to 3.7 pF. The total capacitance for the“01111” M−1 code is equal to 62.9 pF.

FIG. 13 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total inductance as a function ofthe N−1 code. As shown in FIG. 13, for center frequencies in the UHFspectrum, a lower N−-1 code (i.e., overall inductance) yields a highercenter frequency. FIG. 13 also depicts the relationship between the N−1code and center frequency of the LC bank filter (and the desiredchannel) for each value of the M code. For example, curve 870 shows therelationship between the center frequency of an LC bank filter and thetotal inductance as a function of the N−1 code when M is equal to 7.

As discussed above, in order to maximize the Q factor, only certaincombinations of the N code and M code are used. FIG. 14 depictsrelationships between the selected N−1 code and center frequency forvarious combinations of the M code. Note that the relationship betweenthe center LC bank filter frequency and the N−1 code is now confined tothe center of a graph. This relationship minimizes the Q factor, andoptimizes the response of the LC filter bank.

FIGS. 15a and b show the information for selecting inductors in an LBank during UHF tuning. The first column of FIGS. 15a and b lists thetotal inductance, which ranges from 2.85 nH to 91.2 nH, for the L bank.The second column identifies an N−1 code to obtain the correspondinginductance values of column one. For example, a decimal N−1 code of 27yields a total inductance of 3.3 nH. Column 3 of FIGS. 15a and b showsvalid selections for the N−1 code based on the frequency and the valueof the M code. For example, in column 3, a list of frequencies is shownfor valid selections of the N−1 code when the M code is equal to 1.Specifically, column 3 shows that for these frequencies and N−1 code,the M code has a valid range between 11 and 32 (decimal).

FIG. 16 illustrates various resistances for selected capacitances of theC bank. The third column of FIG. 16 lists the resistance, R_(MOS), fromthe MOS transistor for the corresponding capacitor. For example, C2 hasa R_(MOS) resistance of 0.66 ohms. The fourth column lists the totalseries resistance, r_(s), from selecting the corresponding capacitor.For example, C4 has a total series resistance, r_(s), of 1.8 ohms. Thecircuit selects combinations of capacitors from a C bank to reduce thetotal series resistance and thus maximize the Q factor.

FIG. 17 is a timing diagram that shows timing for tuning the LC filterbank in accordance with one embodiment. As shown in FIG. 17, there arefive operations to tune the LC filter bank for a channel in the VHFspectrum. For this embodiment, a timing signal has a frequency of 31.25kHz. First, a frequency measurement of the LC oscillator frequency ismade. As shown in FIG. 17, the LC oscillator frequency measurementoccurs in a single 16 micro second cycle. As described in FIG. 4a, anoffset, A, which indicates the difference between the LC oscillatorfrequency and the desired frequency, is calculated. The process tocalculate the offset, an iterative process, occurs over sixty-four (64)steps (i.e., the value for A is determined in a loop that consists of nomore than 64 iterations). As shown in FIG. 17, each step occurs within16 microseconds.

When tuning the circuit for a desired channel in the VHF spectrum,inductors for the inductor bank are selected first. In one embodiment,the process to select the N code occurs in no more than 32 steps. Again,each step occurs within a 16 micro second cycle.

After inductors for the L bank are selected, a new offset, A, based onthe selected inductor bank, is calculated (see FIG. 4b). As shown inFIG. 17, calculation of the new offset, A, occurs within no more than 64steps.

The fifth operation shown in FIG. 17 selects capacitors from the C.bank. As shown in FIG. 17, the process to select the M code occurs in nomore than 16 steps, with each step having a period of 16 micro seconds.As discussed above in conjunction with FIGS. 11a and 11 b, tuning forchannels in the UHF spectrum involves selecting the M code (tuning thecapacitor bank) and then selecting the N code (tuning the inductorbank).

FIG. 18 illustrates one embodiment for a functional comparator circuitused in one embodiment for tuning the LC filter bank. A comparatorcircuit 1100 is used to evaluate expressions. In general, functionalcomparator circuit 1100 calculates expressions using analog currents.Specifically, for this embodiment, functional comparator circuit 1100evaluates the expressions:

CF _(LC) ² =A ²(1/L)  (1)

CF _(Ch) ² =A ²(1/L)  (2)

CF _(ch) ^(1.5)=2(A ^(1.5)(1/L))  (3)

F _(ch) ^(1.5) C=A ^(1.5)(1/L)  (4)

The above expressions may also be written as:

C/A ²=(1/L)/F _(LC) ²  (1)

C/A ²=(1/L)/F _(ch) ²  (2)

C/2A ^(1.5)=(1/L)/F _(ch) ^(1.5)  (3)

C/A ^(1.5)=(1/L)/F _(ch) ^(1.5)  (4)

The left-hand side of the above expressions (i.e., C/A², C/2A^(1.5), andC/A^(1.5)) are generated using transistors 1112, 1114, and 1118,switches 1120 and 1122, current sources 1130 and 1140 and calculator1200. Switches 1120 and 1122 are set to select either the 1.5 or 2.0exponent for the offset variable, A. For example, to evaluate theexpression C/A^(1.5), switch 1120 is set to couple the current, I_(1.5),for input to calculator 1200.

In one embodiment, current source 1130 is coupled to a digital to analogconverter to convert the digital M code value to an analog current,I_(c). The analog current, I_(c), represent the capacitance and the Cbank. The current sources 1140, also coupled to a digital to analogconverter, converts the digital offset value, A, to an analog current,I_(A). The output of calculator 1200, V_(out), is input to comparator1110. The V_(out) voltage represents a value for the left-handexpression.

The right hand side of the above expressions (i.e., (1/L)/F_(ch) ^(1.5),(1/L)/F_(LC) ², and (1/L)/F_(ch) ^(1.5)) are generated using transistors1102, 1104, and 1106, switches a 1108 and 1110, and current sources 1109and 1111. Switches 1108 and 1110 are used to select the appropriateexponent for the frequency. For example, if the current expression forevaluation is (1/L)/F_(LC) ², then switch 1110 is set. The currentsource 1111 generates an analog current proportional to the inductorvalue for the L bank. In one embodiment, the current source 1111 iscoupled to an output from a digital to analog converter that convertsthe digital value of the N code to an analog current. The current source1109, also coupled to a digital to analog converter, converts thefrequency (LC oscillator frequency or channel code frequency) to ananalog current. The output of calculator 1200 generates a voltage,V_(out), for the right hand expression.

The left-hand expression and right hand expression are input tocomparator 1110. The comparator 1110 compares the V_(out), generated bythe left-hand side of the expression, with the V_(out) generated by theright hand side of the expression.

FIG. 19 illustrates one embodiment for a calculator used in thefunctional comparator circuit of FIG. 18. The calculator circuitreceives, as inputs, the weighted currents for frequency or offset, aswell as the analog currents for inductance or capacitance. As shown inFIG. 19, the weighted currents for frequency or offset, A, with anexponent of two is input to transistors 1204, 1206, and 1210. Theweighted currents for frequency or offset, A, with an exponent of 1.5are input to transistors 1212 and 1222. The analog current forinductance or capacitance is input to the base of bipolar transistor1224. In turn, comparator circuit generates a voltage in accordance withthe following expression:${Vout} = {V\quad t\quad {\ln \left( {\frac{1}{Is}\frac{Inum}{Iden}} \right)}}$

FIG. 20 illustrates a plurality of frequency responses for oneembodiment of the LC filter bank. As shown in FIG. 20, the LC filterbank, through selection of different inductors and capacitors, generatesa wide range of frequency responses. The LC filter bank is a tunablefilter that may be used in circuits that operate on a wide range offrequencies, such as a television receiver.

The LC filter banks filters (i.e., discrete passive filters) enhance theperformance of the tuner circuit. The use of a continuous or activefilter requires a power supply voltage (e.g., V_(cc)). The power supplyvoltage exhibits a ripple due to noise on the voltage supply line. Thisripple voltage, in turn, causes unacceptable frequency responsecharacteristics on the output of the continuous amplifier. Thus, the useof the discrete or passive filters in the receiver isolate the signalfrom ripple voltage, thereby improving signal quality.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method for tuning an inductive (“L”) andcapacitive (“C”) filter based on a desired frequency, said methodcomprising the steps of: providing at least one inductive (“L”) bankcomprising a plurality of inductors, said at least one of said inductorsin said L bank being selected for said LC filter by an N code; providingat least one capacitive (“C”) bank comprising a plurality of capacitors,said at least one of said capacitors in said C bank being selected forsaid LC filter by an M code; determining a first offset between aninitial resonant frequency of said LC filter and a desired frequency forwhich said LC filter is tuned; determining an N code, to selectivelycouple at least one inductor in said L bank, based on an initial valuefor said M code, said desired frequency, and said first offset;selecting at least one inductor in said L bank based on said N code;determining a second offset between a resonant frequency of said LCfilter and a desired frequency for which said LC filter is tuned;determining an M code, to selectively couple at least one capacitor insaid C bank, based on said value for said N code, said desiredfrequency, and said second offset; and selecting at least one capacitorin said C bank based on said M code.
 2. The method as set forth in claim1, wherein the steps of determining a first and second offset comprisesthe step of evaluating the expression: CF _(LC) ² =A ²(1/L), wherein, Arepresents said first and second offset, C represents capacitance ofsaid C bank, L represents inductance of said L bank, and F_(LC)represents said resonant frequency of said LC filter.
 3. The method asset forth in claim 2, wherein the step of evaluating the expressioncomprises the steps of: setting an initial value for said first andsecond offsets; determining whether said expression evaluates to true;setting a new value for said first and second offset if said expressiondoes not evaluate to true; and repeating the steps of determiningwhether said expression evaluates to true and setting a new value untilsaid expression evaluates to true.
 4. The method as set forth in claim3, wherein the step of determining whether said expression evaluates totrue comprises the steps of: converting said F_(LC) to a digital value;storing a digital value for said A value; storing an M code for said Cvalue; storing an N code for said L value; converting said F_(LC), A, Mcode and N code to an analog current; calculating said F_(LC) ²expression and said A² expression in a calculator; generating saidCF_(LC) ² expression and said A²(1/L) expression; and comparing saidCF_(LC) ² expression with said A²(1/L) expression.
 5. The method as setforth in claim 1, wherein the steps of determining an N code comprisesthe step of evaluating the expression: CF _(ch) ² =A ²(1/L), wherein, Arepresents said first offset, C represents capacitance of said C bank, Lrepresents inductance of said L bank, and F_(ch) represents said desiredfrequency.
 6. The method as set forth in claim 5, wherein the step ofevaluating the expression comprises the steps of: setting an initialvalue for inductance of said L bank; determining whether said expressionevaluates to true; setting a new value for said inductance of said Lbank if said expression does not evaluate to true; and repeating thesteps of determining whether said expression evaluates to true andsetting a new value until said expression evaluates to true.
 7. Themethod as set forth in claim 6, wherein the step of determining whethersaid expression evaluates to true comprises the steps of: convertingsaid F_(ch) to a digital value; storing a digital value for said Avalue; storing an M code for said C value; storing an N code for said Lvalue; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ² expression and said A² expression ina calculator; generating said CF_(ch) ² expression and said A²(1/L)expression; and comparing said CF_(ch) ² expression with said A²(1/L)expression.
 8. The method as set forth in claim 1, wherein the steps ofdetermining an N code comprises the step of evaluating the expression:CF _(ch) ^(1.5) =A ^(1.5)(1/L), wherein, A represents said secondoffset, C represents capacitance of said C bank, L represents inductanceof said L bank, and F_(ch) represents said desired frequency.
 9. Themethod as set forth in claim 8, wherein the step of evaluating theexpression comprises the steps of: setting an initial value forcapacitance of said C bank; determining whether said expressionevaluates to true; setting a new value for said capacitance of said Cbank if said expression does not evaluate to true; and repeating thesteps of determining whether said expression evaluates to true andsetting a new value until said expression evaluates to true.
 10. Themethod as set forth in claim 9, wherein the step of determining whethersaid expression evaluates to true comprises the steps of: convertingsaid F_(ch) to a digital value; storing a digital value for said Avalue; storing an M code for said C value; storing an N code for said Lvalue; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ^(1.5) expression and said A^(1.5)expression in a calculator; generating said CF_(ch) ^(1.5) expressionand said A^(1.5)(1/L) expression; and comparing said CF_(ch) ^(1.5)expression with said A^(1.5)(1/L) expression.
 11. The method as setforth in claim 1, wherein said desired frequency comprises a frequencyin a very high frequency (“VHF”) spectrum for television tuning.
 12. Amethod for tuning an inductive (“L”) and capacitive (“C”) filter basedon a desired frequency, said method comprising the steps of: providingat least one inductive (“L”) bank comprising a plurality of inductors,said at least one of said inductors in said L bank being selected forsaid LC filter by an N code; providing at least one capacitive (“C”)bank comprising a plurality of capacitors, said at least one of saidcapacitors in said C bank being selected for said LC filter by an Mcode; determining a first offset between an initial resonant frequencyof said LC filter and a desired frequency for which said LC filter istuned; determining an M code, to selectively couple at least onecapacitor in said C bank, based on an initial value for said N code,said desired frequency, and said first offset; selecting at least onecapacitor in said C bank based on said M code; determining a secondoffset between a resonant frequency of said LC filter and a desiredfrequency for which said LC filter is tuned; determining an N code, toselectively couple at least one inductor in said L bank, based on saidvalue for said M code, said desired frequency, and said second offset;and selecting at least one inductor in said L bank based on said N code.13. The method as set forth in claim 12, wherein the steps ofdetermining a first and second offset comprises the step of evaluatingthe expression: CF _(LC) ² =A ²(1/L), wherein, A represents said firstand second offset, C represents capacitance of said C bank, L representsinductance of said L bank, and F_(LC) represents said resonant frequencyof said LC filter.
 14. The method as set forth in claim 13, wherein thestep of evaluating the expression comprises the steps of: setting aninitial value for said first and second offsets; determining whethersaid expression evaluates to true; setting a new value for said firstand second offset if said expression does not evaluate to true; andrepeating the steps of determining whether said expression evaluates totrue and setting a new value until said expression evaluates to true.15. The method as set forth in claim 14, wherein the step of determiningwhether said expression evaluates to true comprises the steps of:converting said F_(LC) to a digital value; storing a digital value forsaid A value; storing an M code for said C value; storing an N code forsaid L value; converting said F_(LC), A, M code and N code to an analogcurrent; calculating said F_(LC) ² expression and said A² expression ina calculator; generating said CF_(LC) ² expression and said A²(1/L)expression; and comparing said CF_(LC) ² expression with said A²(1/L)expression.
 16. The method as set forth in claim 12, wherein the stepsof determining an M code comprises the step of evaluating theexpression: CF _(ch) ^(1.5)=2(A ^(1.5)(1/L)), wherein, A represents saidfirst offset, C represents capacitance of said C bank, L representsinductance of said L bank, and F_(ch) represents said desired frequency.17. The method as set forth in claim 16, wherein the step of evaluatingthe expression comprises the steps of: setting an initial value forcapacitance of said C bank; determining whether said expressionevaluates to true; setting a new value for said capacitance of said Cbank if said expression does not evaluate to true; and repeating thesteps of determining whether said expression evaluates to true andsetting a new value until said expression evaluates to true.
 18. Themethod as set forth in claim 17, wherein the step of determining whethersaid expression evaluates to true comprises the steps of: convertingsaid F_(ch) to a digital value; storing a digital value for said Avalue; storing an M code for said C value; storing an N code for said Lvalue; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ^(1.5) expression and said A^(1.5)expression in a calculator; generating said CF_(ch) ^(1.5) expressionand said 2(A^(1.5)(1/L)) expression; and comparing said CF_(Ch) ^(1.5)expression with said 2(A^(1.5)(1/L)) expression.
 19. The method as setforth in claim 12, wherein the steps of determining an N code comprisesthe step of evaluating the expression: CF _(Ch) ^(2.0) =A ^(2.0)(1/L),wherein, A represents said second offset, C represents capacitance ofsaid C bank, L represents inductance of said L bank, and F_(ch)represents said desired frequency.
 20. The method as set forth in claim19, wherein the step of evaluating the expression comprises the stepsof: setting an initial value for inductance of said L bank; determiningwhether said expression evaluates to true; setting a new value for saidinductance of said L bank if said expression does not evaluate to true;and repeating the steps of determining whether said expression evaluatesto true and setting a new value until said expression evaluates to true.21. The method as set forth in claim 20, wherein the step of determiningwhether said expression evaluates to true comprises the steps of:converting said F_(ch) to a digital value; storing a digital value forsaid A value; storing an M code for said C value; storing an N code forsaid L value; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ^(2.0) expression and said A^(2.0)expression in a calculator; generating said CF_(ch) ^(2.0) expressionand said A^(2.0)(1/L) expression; and comparing said CF_(ch) ^(2.0)expression with said A^(2.0)(1/L) expression.
 22. The method as setforth in claim 12, wherein said desired frequency comprises a frequencyin a ultra high frequency (“UHF”) spectrum for television tuning.
 23. Acircuit comprising: at least one inductive (“L”) bank comprising aplurality of inductors; at least one capacitive (“C”) bank comprising aplurality of capacitors; a plurality of switches, coupled to said L bankand said C bank, said switches for selecting at least one inductor insaid L bank based on an N code, and for selecting at least one capacitorin said C bank based on an M code; and a comparator circuit fordetermining a first offset between an initial resonant frequency of saidLC filter and a desired frequency for which said LC filter is tuned, fordetermining an N code, to selectively couple at least one inductor insaid L bank, based on an initial value for said M code, said desiredfrequency, and said first offset, for determining a second offsetbetween a resonant frequency of said LC filter and a desired frequencyfor which said LC filter is tuned, and for determining an M code, toselectively couple at least one capacitor in said C bank, based on saidvalue for said N code, said desired frequency, and said second offset.24. A circuit comprising: at least one inductive (“L”) bank comprising aplurality of inductors; at least one capacitive (“C”) bank comprising aplurality of capacitors; a plurality of switches, coupled to said L bankand said C bank, said switches for selecting at least one inductor insaid L bank based on an N code, and for selecting at least one capacitorin said C bank based on an M code; and a comparator circuit fordetermining a first offset between an initial resonant frequency of saidLC filter and a desired frequency for which said LC filter is tuned, fordetermining said M code, to selectively couple at least one capacitor insaid C bank, based on an initial value for said N code, said desiredfrequency, and said first offset, for determining a second offsetbetween a resonant frequency of said LC filter and a desired frequencyfor which said LC filter is tuned, and for determining said N code, toselectively couple at least one inductor in said L bank, based on saidvalue for said M code, said desired frequency, and said second offset.